1. Field of the Invention
This invention relates to tools used downhole, and particularly tools useful in very deep and/or very tortuous wells.
2. Description of the Related Art
Tractor devices are used when drilling for minerals in the earth when it becomes difficult or uneconomical to use traditional, gravity-assisted bottom hole assemblies. In high inclination or tortuous wells it can be difficult to push a drillstring, casing string or workstring along the wellbore due to excessive friction. This can be especially problematic with coiled tubing where the force that can be applied is limited by helical or sinusoidal lockup where the tubing string locks in the wellbore and any additional force applied from surface is not transferred to the bottom of the string. Various downhole tractor devices may be used to assist in propelling tubulars along a wellbore and can be especially useful for coiled tubing applications.
Downhole tractors typically rely on contact with casing or the wellbore to pull the tubing string along the borehole. Although this technique works acceptably in cased hole sections, it is less successful in an open or unlined hole because of inconsistent hole diameter and inadequate formation strength. Typical downhole tractor devices have mechanisms which engage the borehole wall with gripper-type devices, and then push downward on the drill string to force the drill bit into the formation being drilled. Because it is difficult to provide bearing assemblies in these tractor mechanisms that transfer the thrust to a rotating drill string, most tractor devices rely upon a drilling motor mounted in the drill string below the tractor to rotate the drill bit. To make the drill bit advance, the tractor mechanism pushes upon the drill pipe until the device reaches the end of its stroke.
When the end of the stroke is reached, the tractor device typically pulls the drill bit upward as far as its stroke allows and then releases from the borehole wall and is lowered downward or is ‘walked’ downward by pushing upon a second gripper assembly mounted above. As a result the device moves downward in the hole in a series of start/stopped increments. By way of example, two mechanisms of this type are described in U.S. Pat. Nos. 2,946,578 and 7,121,364.
Others tractor device use wheels or tracks to contact the bore wall and provide a continuous driving force.